Implantable medical devices (IMD's), for example, cardiac pacemakers, defibrillators, neurostimulators and drug pumps, which include electronic circuitry and battery elements, require a housing to contain and hermetically seal these elements within a body of a patient. Many of these IMD's include one or more electrical feedthrough assemblies to provide electrical connection between the elements contained within the housing and components of the IMD external to the housing, for example, sensors and/or electrodes and/or lead wires mounted on an exterior surface of the housing, or electrical contacts housed within a connector module, which is mounted on the housing to provide coupling for lead wires.
A feedthrough assembly for an IMD can be unipolar or multipolar (e.g. bipolar etc.). A unipolar feedthrough assembly includes a single feedthrough member, or pin, that extends from an interior to an exterior of the housing through a ferrule, while a multipolar feedthrough assembly includes a plurality of such feedthrough members extending through a single ferrule. In each type of assembly, the feedthrough pin(s) is/are electrically isolated from the ferrule, and, in the case of the multipolar assembly, from one another, by an insulator element, for example, glass or ceramic, that is mounted within the ferrule and surrounds the feedthrough pin(s). The insulator is hermetically sealed to the ferrule and to the feedthrough pin(s), typically, by a braze joint.
To reduce the effects of stray electromagnetic interference (EMI) signals that may be collected by lead wires electrically coupled to the feedthrough pins, it is known to incorporate, within feedthrough assemblies, capacitive elements for high frequency filtering. A filtered feedthrough assembly may be formed by mounting the capacitive element within the ferrule after sealing the insulator element to the ferrule and the feedthrough pin(s); the capacitive element typically includes an insulative base, for example, a ceramic monolith, in which electrode plates are embedded, otherwise known as a discoidal-type capacitor. A first set of the electrode plates are electrically coupled to a conductive layer overlaying an inner surface of the capacitive element, and a second set of the electrode plates are electrically coupled to another conductive layer overlaying an outer surface of the capacitive element. Typically, a conductive material applied between the inner surface of the capacitive element and the pin, and between the outer surface of the capacitive element and the ferrule, forms an electrical coupling between the first set of electrode plates and the pin, and between the second set of electrode plates and the ferrule.
There is a need in filtered feedthrough assemblies, such as those described above, to isolate, within the ferrule, the conductive material, which forms the electrical couplings of the capacitive element, from the brazed joints of the insulator element. In the past this need has been met by applying an insulative barrier of a non-conductive thermosetting adhesive, between the capacitive element and the insulator element, prior to applying the conductive material. However, this approach requires careful control of adhesive volumes in order to prevent excess adhesive from wicking into those areas between the ferrule and the capacitive element, and between the feedthrough pin and the capacitive element, where a presence of subsequently applied conductive material is intended. Thus, in order to provide for this isolation, while simplifying the assembly process, there is a need for new isolation methods and materials.